1. Field of the Invention
The present invention relates to a drive force distribution apparatus for a hybrid vehicle having an engine for driving a pair of first drive wheels and an electric motor for driving a pair of second drive wheels.
2. Description of the Related Art
Conventionally, a control apparatus for this kind of hybrid vehicle is known from Japanese Patent Laid-open No. 2000-79833, for example. In this hybrid vehicle, front wheels are driven by an engine connected thereto, and rear wheels are driven by an electric motor connected thereto through a differential for the rear wheels. During deceleration of the hybrid vehicle, the electric motor functions as a generator for generating electric power, thereby recovering running energy as electrical energy and charging the electrical energy into a capacitor. During any running conditions other than the deceleration, the electric motor is driven as required by the electrical energy charged in the capacitor.
As general vehicle behavior stability control, it is known that a throttle opening is controlled so as to suppress the slip of drive wheels upon detection of the slip according to information from wheel speed sensors, an accelerator opening sensor, a throttle opening sensor, etc., thereby regulating an engine output. It is also known that a brake oil pressure applied to a wheel locked upon tire locking during braking is removed to thereby avoid the tire locking. Further, a vehicle behavior stability control system for preventing lateral slip of a vehicle during turning of the vehicle has been put to practical use, wherein a brake oil pressure applied to a specified wheel is increased and brake forces for wheels are individually controlled according to information such as a steering angle, lateral G, and yaw rate. Further, in a four-wheel drive vehicle, there has been proposed a drive system intended for stabilization of vehicle behavior by suitably distributing front and rear drive forces.
According to the hybrid vehicle described in the above-mentioned publication, the electric motor is connected through the differential to the rear wheels. Accordingly, the electric motor must always run together with the rotation of the rear wheels, and the running speed of the electric motor is higher than the rotational speed of the rear wheels because of the reduction ratio of the differential. It is therefore necessary for the electric motor to continuously run at high speeds during high-speed running of the vehicle, so that the durability against such high-speed continuous running is required. As a result, the electric motor is necessarily enlarged in size to ensure the durability, inviting an increase in manufacture cost. Further, since the right and left rear wheels are connected through the differential to the electric motor, the same torque is transmitted to the right and left rear wheels, and the vehicle behavior stability control is therefore insufficient during turning of the vehicle or upon slippage of the rear wheels.
Further, in a general vehicle capable of arbitrarily setting an engine output, only drive wheels (front wheels in an FF vehicle or rear wheels in an FR vehicle) can be controlled, and these right and left drive wheels are connected through a differential to the engine. Accordingly, a drive force of the engine cannot be suitably distributed to the right and left drive wheels. In vehicle lateral slip prevention control, a brake force is applied to a specified wheel to thereby stabilize the vehicle behavior, so that a part of the engine output is discarded as heat through a hydraulic brake.
It is therefore an object of the present invention to provide a drive force distribution apparatus for a hybrid vehicle which can arbitrarily control the distribution of a drive force to a pair of drive wheels connected to an electric motor according to a running condition of the vehicle.
In accordance with an aspect of the present invention, there is provided a drive force distribution apparatus having an engine for driving a pair of first drive wheels and an electric motor for driving a pair of second drive wheels. The drive force distribution apparatus includes a first clutch interposed between the electric motor and one of the second drive wheels; a second clutch interposed between the electric motor and the other of the second drive wheels; a battery for supplying electric energy to the electric motor and storing electric energy regenerated by the electric motor; a vehicle speed sensor for detecting a vehicle speed; an accelerator opening sensor for detecting an accelerator opening; a brake depression force sensor for detecting a brake depression force; a steering angle sensor for detecting a steering angle of a steering wheel; and a yaw rate sensor for detecting a yaw rate.
The drive force distribution apparatus further includes first means for deciding the distribution between a drive force for driving the first drive wheels and a drive force for driving the second drive wheels according to the vehicle speed detected, the accelerator opening detected, the brake depression force detected, a state of the battery, and a weight distribution of the vehicle; second means for deciding the distribution between a drive force for driving one of the second drive wheels and a drive force for driving the other of the second drive wheels according to the vehicle speed detected, the steering angle detected, and the yaw rate detected; and clutch control means for controlling a degree of engagement of each of the first and second clutches according to the distribution decided by the second means.
According to the present invention, the distribution between the drive force of the engine for driving the first drive wheels and the drive force of the electric motor for driving the second drive wheels can be properly made by the first means according to the detected vehicle speed, accelerator opening, brake depression force, battery state, and vehicle weight distribution. During deceleration of the vehicle, the running energy of the vehicle can be regenerated as electrical energy by the electric motor according to the brake depression force, battery state, etc., and the regenerated energy can be used for driving of the vehicle, thereby improving a fuel consumption. Further, the distribution between the drive force for driving one of the second drive wheels and the drive force for driving the other second drive wheel is decided by the second means according to the detected vehicle speed, steering angle, and yaw rate. Then, the degree of engagement of each of the first and second clutches is controlled by the clutch control means according to the distribution decided by the second means, and the drive force of the electric motor is distributed to the second drive wheels (the right and left drive wheels), thereby allowing the stabilization of vehicle behavior such as prevention of lateral slip during turning of the vehicle.
The second means includes means for deciding a calculated lateral G from the vehicle speed and the steering angle; means for deciding a lateral distribution coefficient KLR from the calculated lateral G and the vehicle speed; means for calculating a steering angle yaw rate and a lateral G yaw rate; means for calculating a corrected torque value KTQ according to the steering angle yaw rate calculated and the lateral G yaw rate calculated; and means for calculating the drive forces for driving the second drive wheels according to the lateral distribution coefficient KLR and the corrected torque value KTQ. Preferably, when the sign of the drive force calculated on one of the second drive wheels is different from the sign of the drive force calculated on the other second drive wheel, either of the drive forces is set to 0. More preferably, the drive force distribution apparatus further includes a G sensor for detecting a lateral G, and the second means decides the distribution between the drive forces for driving the second drive wheels according to the vehicle speed detected, the steering angle detected, the yaw rate detected, and the lateral G detected.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.